This invention relates generally to radio telephone communication systems having a base station and a remote station and specifically to a method and means for operating such systems to produce enhanced security.
Presently the Federal Communications Commission authorizes a very limited number of FM broadcast channels for two-way wireless telephone communications. Such two-way telephone systems provide operation substantially similar to that experienced with conventional wired telephones by using two different frequency carrier signals for each installation. With the arrangement, a person speaking into the microphone or hand set of the remote station can hear his own voice in his receiver because the signal is relayed back to him by the base station as well as sent out along the telephone lines. This type of operation requires one carrier for transmission and another carrier for reception.
Because of the small number of allocated carrier frequencies, the number of combinations of transmitter and receiver carrier frequencies is extremely limited. While in many instances this is not a serious problem, generally because the range of the transmitters is quite short and the actual number of systems in use is still small, the "privacy" of these telephone systems leaves much to be desired. Further, there have been instances where unauthorized users have managed to "contact" a base station with a wireless transmitter of proper frequency and place telephone calls thereover. Such calls obviously are charged to the person to whom the base station is registered, rather than to the actual user.
In an effort to thwart "eavesdropping" and unauthorized use, manufacturers have built systems wherein an address code is needed to contact the base station and vice versa. One such system is said to provide a binary address yielding 256 different code combinations, a specific one of which must be transmitted by a remote unit to contact its base station. However, in all such systems, the limited number of carrier frequencies assigned makes it relatively easy to eavesdrop, and while an identification code may minimize unauthorized calls over a base station, the privacy of such telephone systems is still easily compromised because of the retransmission feature mentioned above, which makes both parties conversation susceptible to eavesdropping.
Consequently, there is a need in the art for a two-way radio telephone communication system of enhanced privacy.